The idea of permitting travel only to those with a vaccine certificate assumes that a vaccine is readily available to all who want it for modest cost.
If we assume a vaccine is approved by 31 Dec, and each of the 1,200 NHS Primary Care Networks can provide the hoped-for 1,000 shots per week, then a vaccine that requires 2 shots will achieve (on average) 600,000 people per week. There are 12 million people over 65 in the UK - so it would take until the end of May before they have immunity. It will likely take until November next year before all the over 50s are vaccinated. The UK doesn't even have a plan to vaccinate the under 50s - it would probably take until the end of 2022 before people in their 20s were vaccinated
Are we going to effectively ban cross-border travel for anyone aged under 50 for at least the next 12 months ? If so, the airline and travel industry is going to end up in a very troubled state